ca01 only detailed ToC 1..24

36 Science of Synthesis 1.1 Organometallic Complexes of Nickel
1.1.1.4.2 Variation2:
Triethylborane-Mediated Reactions
The intermolecular process between simple dienes and aldehydes is reported by Tamaru.
[31] Triethylborane is employed as the reducing agent, and yields are good for a variety
of substituted electron-rich and electron-poor dienes. Interestingly, reactions employing
triethylborane and bis(acetylacetonato)nickel(II) (1) produce 4,5-unsaturated alcohols
(Scheme 9), whereas reactions employing bis(ç 4 -cycloocta-1,5-diene)nickel(0) (2), triphenylphosphine,
and triethylsilane produce 3,4-unsaturated silyl ethers. The mechanistic basis
for this reversal of regioselectivity has not been established.
Scheme 9 Reductive Coupling with Triethylborane [31]
O
BEt3 (2.4 equiv)
OH
CO2Me +
H Ph
10 mol% Ni(acac) 2 1
91%
Ph
CO2Me 14 15
Methyl (2R*,4E)-2-[(R*)-Hydroxy(phenyl)methyl]hex-4-enoate (15): [31]
Into a N 2-purged flask containing [Ni(acac) 2](1; 12.8mg, 0.05 mmol) were introduced successively
freshly dried (Na benzophenone ketyl) THF (3 mL), methyl (2E,4E)-hexa-2,4-dienoate
(14; 2.52 g, 20 mmol), PhCHO (530 mg, 5 mmol), and 1 M BEt 3 in hexane (12.0 mL)
via a syringe. The homogeneous mixture was stirred at rt for 66 h until the PhCHO disappeared
completely. After dilution with EtOAc (50 mL), the mixture was washed successively
with 2 M HCl, sat. NaHCO 3, and sat. NaCl, and then dried (MgSO 4), and concentrated in
vacuo. The residual oil was subjected to column chromatography (silica gel, hexanes/
EtOAc 16:1) to give an analytically pure sample of 15; yield: 1.07 g (91%).
1.1.1.5 Method 5:
1,4-Dialkylationof Dienes
Studies by Chang demonstrate that two molecules of an iodoalkene (e.g., 17) readily add
across a 1,3-diene (e.g., 16) to give predominantly symmetrical 1,4-addition products such
as (18) (Scheme 10). [32] Nickel(0) is consumed in the reaction; however, the use of zinc
powder as a reductant allows the nickel to be used in catalytic amounts. Generally, a cis
orientation of the internal double bond is obtained. With cyclic dienes, a cis orientation of
the two alkenyl substituents is obtained.
Scheme 10 1,4-Dialkylation of a Conjugated Diene [32]
16
+
O
NiBr2, Zn
Ph3P, MeCN
O O
82%
I
17 18
2,3-Dimethyl-1,4-bis(3-oxocyclohex-1-enyl)but-2-ene (18): [32]
To a 50-mL side-arm flask were added NiBr 2 (0.0204 g, 0.100 mmol), Ph 3P (0.0262 g,
0.100 mmol), and Zn powder (0.082 g, 1.25 mmol). The system was purged with N 2 three
times. MeCN (0.50 mL), 3-iodocyclohex-2-en-1-one (17; 0.222 g, 1.00 mmol), and 2,3-dimethylbuta-1,3-diene
(16; 0.246 g, 3.00 mmol) were added by syringe, and the soln was
stirred at 808C for 2 h. During the reaction the soln gradually turned from yellow to red.
At the end of the reaction the system was filtered through Celite. The filtrate was concen-